A rotator core and stator core of an electric rotating machine are formed of layers of thin plates such as silicon-steel plate. Generally, thin plates are joined by welding. However, since a convex projection arises in the welding position, the gap between the cores cannot be made small. That is, if the gap is made small, the core facing the convex projection will interfere with the convex projection.
Hence, the opposite side of a magnetic pole-face is welded so that there is no weld part with a surface (magnetic pole-face) facing another core.